The invention relates to a flange seal, having a self-contained sealing frame disposed between two flanges which are to be connected in a pressure-tight manner. The sealing frame rests on the flanges with its sealing faces at either side, and a groove extends around and along each sealing face. A self-contained sealing element, preferably of metal, is fitted into the groove and may protrude somewhat beyond the sealing face.
German Patent No. P 28 17 666.1 describes an electrically insulating tube fitting for overpressure or vacuum systems, particularly in accelerator engineering, where individual tubular line sections must be connected to one another in a pressure-tight manner while being electrically insulated from one another; the joints must also be able to withstand high temperatures. Between the flanges to be connected, there is an annular disc of polymeric plastic, acting simultaneously as a sealing body and as an electrical insulator. This plastic may be, for example, a polyimide or aramide resin, which is far superior to metal or ceramic materials in terms of its mechanical strength and especially its wear and creep resistance. The outer diameter of the annular disc is smaller than the outer diameter of the flange, and the protruding flange parts are conventionally screwed together by bolts guided in insulating flange bushings.
Even when the surfaces of conventional insulating materials are machined to an extremely fine degree, a certain surface roughness still remains, making a complete vacuum seal impossible. Vacuum grease is generally used to fill in this roughness; it has the disadvantage, however, that at a very extreme vacuum its vapor pressure causes it to diffuse into the vacuum, thus contaminating the vacuum. As an alternative, therefore, the surfaces have been sealed as well, specifically in the case of insulating annular discs or sealing frames of hardened glass-mesh plates, which in practice cannot be rendered completely poreless and free of capillaries. However, sealing the surfaces is rather expensive and the surfaces are easily damaged, so that repairs involving a new application of resin, with subsequent grinding or polishing by hand, are frequently required. The difficulties associated with the known sealing frames become increasingly severe, the larger the dimensions of the sealing frame.
A flange seal is also known from German Offenlegungsschrift No. 26 16 512, in which grooves are machined into the sealing faces at either side of a sealing frame made of steel; round sealing rings are placed in these grooves and may be made of silver, copper or lead, for example, which are crushed when the flange seal is screwed together. If the sealing frames are to be used again after the flange connection has been opened, then the old sealing rings must be removed and new, not-yet-crushed sealing rings must be put into place. Furthermore, it is not possible with a seal of this kind to produce a connection which provides electrical insulation of the flanges from one another.